Biaxially oriented polyester films in which polyethylene terephthalate, polyethylene-2,6-naphthalate or the like is used are excellent in mechanical properties, heat resistance, dimension stability, chemical resistance, cost efficiency, etc. Therefore, with the advantage of their performance, the biaxially oriented polyester films are employed in various applications. One example of the applications is a thermal transfer ribbon. The thermal transfer recording method has been already used in areas such as a facsimile, bar code printing, etc., because of its excellent cost efficiency, maintenance property, operability, and the like. The use of a sublimation dye in the thermal transfer ink instead of the existing melting pigment allows for properties such as high resolution, high image quality and the like. As a result, the thermal transfer recording method has been used frequently in the area of digital photo printing, etc., and has progressively replaced the conventional method using silver-halide method. In recent years, the quality demanded in the digital photo printing has been increasing. Even in the thermal transfer recording method, the glossiness as good as that of the inkjet recording method is required in printed objects. Therefore, a printing property greater than ever is demanded in a polyester film for a thermal transfer ribbon. In order to improve the printing property, a method to smoothen the surface of the polyester film by decreasing the content of particles contained in the polyester film or reducing the particle size has been discussed so far (Patent Document 1).